5 kW Axpert Inverters in Parallel

[Calvin]

Hi All,

I am looking to go "off-grid" and am considering a system consisting of 

• 36 x 330W Canadian Solar
• 3 x Axpert  5kVA inverters in parallel single phase mode 
• 8 x Pylontech 3.5 kWh batteries (may be expanded later)
• 1 x 5 kW MAC-AFRICA 6.5 kVA Standby Silent Diesel Generator 220 V (for the no-sunshine days)

The bit that I most need help on is the Axpert inverters.  Specifically:

1. When connected in parallel, do they really act as a single unit:

• Are all PV panel outputs connected together, or is a third connected to each inverter?
• Are all the loads connected together, or is one third connected to each inverter?
• If one inverter fails, do they all go down?
• Should my generator be connected to only one unit, or to all three? (I will manage loads when the generator is the only input in order to not overload it)

2. I have read that these Axpert units are quite sensitive to the generator output (slew rates etc).  Does anybody have experience with my proposed setup?

3. I have read various reports about the ability of the Axpert units to communicate with the Pylontech batteries. (Possibly due to older firmware versions).  Anybody have a latest state of play?

4. I understand that the MKS II can run higher PV voltages (than the MKS or King), allowing one to have up to 9 (10?) panels in series.  I want 12 panels so will need at least 2 strings and a combiner box per inverter anyway.  Any thoughts on pros and cons?

5. Given that the MKS and King inverters specify MPPT voltage as 60-115, does that mean that one should have only 2 72 cell panels in a string when using those? (I see many people connect 3 due to max open circuit voltage of 145V)

Any other thoughts / comments, particularly on choosing between MKS / MKS II / King would be welcome.

Thank you in advance.

[Coulomb]

15 hours ago, Calvin said:

1. When connected in parallel, do they really act as a single unit:

Yes, pretty much.

15 hours ago, Calvin said:

• Are all PV panel outputs connected together, or is a third connected to each inverter? 

I think you mean PV inputs. They have to be separate. Not necessarily evenly divided.

• Are all the loads connected together, or is one third connected to each inverter?

The outputs of each inverter, and all the loads, are paralleled. So you don't overload one inverter while the others still have capacity.

• If one inverter fails, do they all go down?

Depends. Most of the time, I'd expect them all to at least drop off. So each should be able to be isolated, battery, output, and inputs (AC in and PV in).

• Should my generator be connected to only one unit, or to all three? (I will manage loads when the generator is the only input in order to not overload it)

I would connect to all three.

2. I have read that these Axpert units are quite sensitive to the generator output (slew rates etc).  Does anybody have experience with my proposed setup?

I don't know your specific generator, but yes they are fussy, and in general inverter based generators work better.

3. I have read various reports about the ability of the Axpert units to communicate with the Pylontech batteries. (Possibly due to older firmware versions).  Anybody have a latest state of play?

It's still unclear at least to me.

4. I understand that the MKS II can run higher PV voltages (than the MKS or King), allowing one to have up to 9 (10?) panels in series.  I want 12 panels so will need at least 2 strings and a combiner box per inverter anyway.  Any thoughts on pros and cons?

6S may be a bit low voltage if they are 60 cell panels.

5. Given that the MKS and King inverters specify MPPT voltage as 60-115, does that mean that one should have only 2 72 cell panels in a string when using those?

Yes.

> (I see many people connect 3 due to max open circuit voltage of 145V)

3S is good if and only if they are 60 cell panels.